Healthy volunteers 18 years of age and older may be eligible for this study. Candidates are screened with a medical history, physical examination, and blood tests. Subjects with hyperlipidemia, impaired glucose tolerance or diabetes and thyroid disease are not eligible for the participation in this study. Qualifying study participants undergo subcutaneous fat tissue microdialysis to assess the effects of T3 and T4 on the activity of fat tissue. The medications isoproterenol and T3 or T4 are injected in fat tissue under the skin of the abdomen, and fluid samples are collected from the area over a 6-12 hour period using the same small needle. Glycerol is then measured to study how the thyroid hormones affect fat metabolism. During this procedure a 10% glucose solution is infused through the catheter in the arm to limit lipolysis due to fasting. Four different sets of experiments are performed with different thyroid hormone doses and/or durations.